Nuclear tests, such as those conducted at the Nevada Test Site (NTS), are often conducted below ground, such as at or below the groundwater table. These tests inject various radionuclides, including tritium, into the groundwater. Groundwater wells have been drilled at and near the NTS to sample the groundwater for radionuclides. The wide range of half-lives of the radionuclides of concern means that subsurface monitoring for some of these constituents will occur for the foreseeable future. Some of these radionuclides and their associated half-lives are: 3H (12.3 yrs), 90Sr (28.8 yrs), 137Cs (30.1 yrs), and 238U (4.47E+9 yrs).
While tritium has a relatively short half-life, tritium is of interest because of its groundwater transport characteristics. Tritium generally does not react with rock and mineral surfaces of an aquifer during groundwater transport. As a consequence, tritium typically moves at the average groundwater velocity and usually is transported ahead of other reactive radionuclides. Accordingly, tritium arrival in a monitoring well can be an indicator of subsequent arrival of other radionuclides.
Current sampling procedures for tritium typically include installing a pump in a monitoring well and removing at least three well volumes of fluid. Such procedures typically require specific fluid management protocols, collecting a sample, removing the pump and pump string, and decontaminating these components. The U.S. Department of Energy (DOE) has estimated that sampling 200 monitor wells for 100 years using current practices would cost over $150 million in 2005 dollars.
Tritium emits a low-energy β-particle (18.6 KeV). Current typical tritium analysis systems are based on liquid scintillation, where a water sample is collected and mixed with a “cocktail” of organic compounds that emit light when struck by the tritium β-particle. A photomultiplier tube amplifies the signal sufficiently to provide an accurate electronic representation of the tritium activity.
Dissolved radioactive ions such as 14C (156.5 KeV), 40K (1,460 KeV), 226Rn (6,000 KeV), and 238U (4,196 KeV) are nearly always present in groundwater. The presence of these ions raises the background radiation level and can reduce the ability to detect tritium against background radiation. Consequently, water must typically be purified to reduce the concentration of these ions to a sufficiently low level.
Although tritium in liquid samples is often of interest, tritium present in vapor form, such as in an underground vapor plume or in the air surrounding a surface site, such as a facility suspected of nuclear activity, may also be of interest. Typical monitoring techniques draw large samples of air to a land-surface mounted cold finger or cold point condenser system to collect vapor, such as soil vapor. These systems can suffer from a number of drawbacks, however. For example, because of the large volume of air actively pumped to the detector, such as from an unsaturated zone, the air can be representative of a large subsurface soil volume. Accordingly, the positional accuracy or precision of such monitoring systems can be greatly diminished. Furthermore, these systems are often expensive, large, and require many kilowatts of power to operate—limiting their use for remote, discrete, or long term radiological monitoring of sites.